In recent years, as a result of the increase in communications traffic, large-capacity optical communication systems have been introduced. In order to increase the communication capacity, WDM that multiplexes and transmits signals having a plurality of wavelengths has been proposed. Increasing the number of wavelengths to be multiplexed allows low-cost operations. Hereafter, the wavelength that carries the optical signal might be described to channel in the WDM system. Furthermore, instead of point-to-point WDM systems, optical add-drop multiplexers (OADM) that adds and/or drops channels at an arbitrary node have been introduced. Thus, a flexible network allowing long distance and high-capacity transmission is formed.
In an optical transmission system, optical-signal-to-noise ratio (OSNR) is generally used as a parameter for indicating the transmission quality of a signal. The value of OSNR limits the transmission distance of the system. FIG. 10A is a diagram illustrating a configuration of a relay section in a communication system. One relay section (which also be called a span) in the communication system includes an optical fiber 1500 having a predetermined length and a repeater 1501. FIG. 10B to FIG. 10D are diagrams illustrating signal levels at spots within the relay section. The horizontal axis indicates the wavelength, and the vertical axis indicates the optical intensity. In the WDM system, a plurality of signals, which are wavelength-division-multiplexed on the optical fiber 1500, are input (Line_in); are reduced by the loss of the optical fiber 1500 having a predetermined length and the reduced signals are input to the repeater 1501 (REP_in); the input signals are amplified by the repeater 1501 and the resulting signal is output (REP_out). In a real system, the configuration of one section illustrated in FIG. 10A is applied in a plurality of sections for signal transmission. During the amplification in the repeater 1501, the ASE noise (amplified spontaneous emission noise) generated in the amplifier may deteriorate the OSNR. The degree of the deterioration may determine the number of relay sections (or the number of spans for transmission), that is, the transmittable distance. The OSNR may be determined by the noise figure (NF) of the amplifier and the input level (Pin) to the amplifier, as expressed in:OSNR=Pin−NF+constant[dB]  [Formula 1];Constant: hυΔf; 
(h: Planck constant, υ: optical frequency, and Δf: normalized bandwidth)
Here, if the NF of the amplifier is a fixed value, the OSNR depends on the input level to the amplifier in the repeater 1501. In other words, as the input level decreases, the OSNR deteriorates proportionately. As a result, the transmission distance decreases. In general transmitting apparatus, various controls are performed so as to prevent the decrease in level as much as possible. Those controls typically include two of level equalization (or gain equalization) and ASE correction.
First of all, the level equalization will be described. FIG. 11A to FIG. 11C are diagrams illustrating the correction of level deviations among channels. It is assumed that there is a level deviation ΔP among channels at the transmission path input point (Line_in) as illustrated in FIG. 11A or the amplifier input point (REP_in) in the repeater 1501 as illustrated in FIG. 11B (where the channel average level is equal to that in FIGS. 10A to 10B). In this case, as illustrated in FIG. 11C, the OSNR of the repeater output (REP_out) of the channels at a lower level, such as a channel in a wavelength λ1, deteriorates on the basis of Formula 1. The channel (in λ1) limits the transmission distance and limits the system performance. This difficulty may be solved by a technology in which the level deviation (which may also be called gain deviation or tilt) among channels is cancelled by a system so as to raise the level of a specific channel with a low OSNR and thus cancel the limit on the transmission distance. (Refer to Japanese Laid-Open Patent Publication No. 2000-209160, for example.)
Next, ASE correction will be described. FIG. 12A to FIG. 12C are diagrams illustrating ASE correction. A WDM amplifier normally monitors the total of output power of an amplifier and controls the power to a target value (which is a required output level for each channel× the number of channels), which may continuously keep the level for each channel. However, as illustrated in FIG. 12A, when the number of the channels is as low as several channels, the proportion of ASE power in the total power to be monitored is relatively high. Thus, when the amplifier output level is controlled without consideration of the proportion of the ASE power, the level for each channel decreases (Line_in (corresponding to one upstream REP_out)). As a result, as illustrated in FIG. 12B, the optical signal level for each channel to be input to a repeater decreases (REP_in), resulting in, as illustrated in FIG. 12C, deterioration of the OSNR arises (REP_out). In order to solve the difficulty, on the ASE correction, there is provided a technique in which input light is demultiplexed into signal light component of the channels and an ASE light component in the amplification band of the amplifier so as to control an amplifier such that the signal light component have a target value. (Refer to Japanese Laid-Open Patent Publication No. 2002-368315, for example.)
If the decrease in signal levels of each channel is prevented by using the above technology, the OSNR deterioration may be prevented. Therefore the extension of the transmission distance may thus be expected. However, even when two measures of the level equalization and ASE correction are taken, there is another factor for deteriorating the OSNR. It is OSNR deterioration caused by variations in ASE levels of the respective amplifiers. FIG. 13A and FIG. 13B are diagrams illustrating an ASE correction when the ASE level is not constant. As illustrated in FIG. 13A, when an ASE profile is not flat, the OSNR deteriorates at a channel where the ASE is relatively high, and the transmission distance is limited. In order to solve the difficulty, as illustrated in FIG. 13B, a pre-emphasis method may be used. By using the method, at a node provided at downstream side, or preferably at a node provided at far end of the downstream on which a receiver is provided, signals having lower OSNRs are amplified in advance so as to keep signal levels constant.
The configuration may eliminate the OSNR deviations among channels after transmission, may prevent the OSNR deterioration of an optical signal at a specific channel and increases the transmission distance.